This invention relates to a lead-antimony alloy, particularly for the grids of the plates of lead-acid storage batteries, preferably of starter batteries for internal-combustion engines.
The grids of known lead-acid storage batteries generally made from a Pb-Sb alloy which contains 4-11% antimony. Antimony imparts the required strength and casting properties to the pure lead, which is inherently very soft and cannot be used to make storage battery grids. In these alloys, the high-cost antimony component appreciably adds to the costs of manufacturing the grids. Besides, the susceptibility of the storage battery to self-discharge increases with an increase of the antimony content. An increase of the antimony content also increases the evolution of gas, and the migration of antimony ions to the negative electrode of the storage battery results in a poisoning of this electrode in operation.
For these reasons it has been attempted to minimize the antimony content of the grids for lead-acid storage batteries.
Attempts to make lead-acid storage battery grids which contain only 1.0-3.5% antimony have failed because the resulting grids are brittle and exhibit a tendency to crack. These phenomena appear to be due to a solidification with formation of coarse dendrites with resulting defects in the casting, such as shrinkage voids, microporosity, etc.
U.S. Pat. No. 2,148,741 discloses a lead-antimony alloy which contains antimony, tin, and arsenic or copper or selenium. An alloy which contains, e-g-, only arsenic and copper has a considerable tendency to crack and embrittle. On the other hand, alloys which contain selenium have the required hardness only if they have an antimony content above 6 %.
According to British Pat. No. 622,512, positive grids for lead-acid storage batteries consist of an alloy of lead, 1-5% antimony, and 0.005-0.5% selenium. The selenium content reduces or eliminates the tendency of the grids to crack as they cool after the casting operation.
As has been described hereinbefore, a cracking of as-cast lead alloys having a low antimony content of 1-3.5% can be avoided but these grids are so soft that they cannot be processed to form finished plates of storage batteries.
British Pat. No. 1,105,548 describes antimony free alloys for grids of lead-acid storage batteries. These alloys consist of lead, tellurium, silver, and arsenic. These alloys are also too soft in general and for this reason have not been successful in storage battery technology.
U.S. Pat. No. 3,801,310 discloses a low-antimony lead alloy for grids of plates of lead-acid storage batteries, which alloy consists of 1.5-3.5% by weight antimony, 0.025-0.20% by weight arsenic, 0.005-0.1% by weight Selenium, and 0.01-0.05% by weight tin, balance lead. When this alloy is used for positive grids of plates of industrial storage batteries and thin positive grids for plates of starter batteries, it is desirable for the alloy to contain 0.025-0.1% by weight silver which is said to stabilize the structure, increase the toughness and improve the resistance to corrosion. According to this disclosure, the addition of tin improves the castability and the casting rate, the addition of selenium prevents cracking, and the addition of arsenic increases the hardness.
On the other hand, this low-antimony lead alloy has the disadvantage that it is contaminated by considerable amounts of dross when melted and cast at the conventional temperature, which are between about 450.degree. C and 550.degree. C. A high contamination by dross results in great difficulties and in losses of metal during the processing of such cast lead alloys.
This invention reduces the contamination by dross whereas the other advantageous properties of the alloy are not adversely affected.